1. Field of the Invention
This invention relates to a method for making a component of a gas seal structure for a gas turbine engine. More particularly, the invention relates to a method for depositing a layer having abrasive characteristics onto the surface of a turbine blade using a brush plating technique.
2. Description of the Prior Art
In the compressor and turbine sections of gas turbine engines, blades rotate about the axis of the engine. The blade tips come in proximity to the inner wall of the engine case, frequently rubbing the case wall, or an abradable seal coated on the case wall. Engine efficiency depends to a significant extent upon minimizing leakage by control of the gas flow in such a manner as to maximize interaction between the gas stream and the moving and stationary blades. A major source of inefficiency is leakage of gas around the tips of the compressor blades, between the blade tips and the engine case. In view of the growing competition in the gas turbine business, the emphasis on closer tolerances, and thus greater efficiencies, is increasing. Although a close tolerance fit may be obtained by fabricating the mating parts to a very close tolerance range, this fabrication process is extremely costly and time consuming. Further, when the mated assembly is exposed to a high temperature environment and high stress, as when in use, the coefficients of expansion of the mating parts may differ, thus causing the clearance space to either increase or decrease. The latter condition would result in a frictional contact between blades and housing, causing elevation of temperatures and possible damage to one or both members. On the other hand, increased clearance space would permit gas to escape between the turbine blade and housing, thus decreasing efficiency.
One means to increase efficiency is to apply a coating of suitable material to the interior surface of the compressor housing, to reduce leakage between the blade tips and the housing. Various coating techniques have been employed to coat the inside diameter of the turbine housing with an abradable coating which can be worn away by the frictional contact of the turbine blade, to provide a close fitting channel in which the blade tip may travel. Thus, when subjecting the coated turbine assembly to a high temperature and stress environment, the blade and the case may expand or contract without resulting in significant gas leakage between the blade tip and the turbine housing. This abradable coating technique has been employed to not only increase the efficiency of the compressor, but to also provide a relatively speedy and inexpensive method for restoring excessively worn turbine engine parts to service.
To extend the life of the blade tips which rub against the abradable seals, abrasive layers are sometimes applied to the blade tip surface by a variety of methods. See, for example, U.S. Pat. 4,802,828, of Rutz et al, which mentions several techniques for providing the abrasive layer on a blade tip, including powder metallurgy techniques, plasma spray techniques, and electroplating techniques; Schaefer et al, U.S. Pat. No. 4,735,656, which teaches application of an abrasive comprising ceramic particulates in a metal matrix by controlled melting and solidification of the matrix metal; or Schaefer et al, U.S. Pat. No. 4,851,188 which teaches a sintering operation for application of an abrasive layer to the tip of a superalloy gas turbine blade.
Electroplating techniques have been previously used for the deposition of abrasive layers to blade tips, as illustrated in Routsis, et al, U.S. Pat. No. 5,074,970, which teaches entrapment of nonconductive particulates within a layer of nickel upon the surface of a compressor blade by submerging the blade tip in a slurry of the particulate in plating solution, and electroplating a layer of nickel about the particulates in contact with the blade tip surface to encapsulate them in place. Similarly, U.S. Pat. No. 4, 608,128, of Farmer et al, relates to deposition of nonconductive particulates on a substrate by applying a nonconductive tape carrying the particles to the blade tip, and electrodeposition of a metallic coating through pores in the tape onto the blade surface and about the abrasive particles, followed by removal of the tape so as to leave the particles on the blade surface, held in place by the electrodeposited metallic coating.
In Stalker et al, U.S. Pat. No. 4,169,020, an abrasive tip is produced by electrodepositing the metal matrix while concurrently entrapping abrasive particles included in the electroplating solution. In this reference, particles are deliberately left protruding from the matrix by limiting the matrix thickness. Wride et al, in U.S. Pat. No. 5,076,897, teach application of a binding coat on the tip of a blade body by electrodeposition, followed by composite electrodeposition of particulate abrasive and an anchoring metal matrix, followed by plating an infill around the abrasive particles.
Brush plating techniques have been taught for the application of various metals to conductive surfaces. For example, Tezuka et al, in U.S. Pat. No. 4,655,881, teach the use of a liquid retaining material, such as a woven fabric, on an anode surface designed to plate opposed parts of a fork-like terminal. In U.S. Pat. No. 4,738,756, Mseitif teaches brush chrome plating using a tank chrome plating solution, rather than more expensive brush plating solutions having higher metal ion concentrations. The Mseitif reference teaches the use of a dielectric porous anode cover.